1. Field of the Invention
The present invention relates to vehicles having a primary transmission and a hydraulic motor that provides a drive assist and, more particularly, a hydraulic circuit including such a hydraulic assist motor.
2. Description of the Related Art
Many large vehicles include a mechanical transmission as the primary driver of the vehicle and use hydraulic assist motors to selectively drive additional wheels. For example, such a vehicle may have rear wheels driven by the mechanical transmission and front steerable wheels that are selectively driven by hydraulic assist motors. When being driven on a paved road, such a vehicle will typically employ only the mechanical transmission. When the vehicle must be driven in poor traction or off-road conditions, e.g., when on a construction site, it can be beneficial to employ the hydraulic assist motors to provide the vehicle with an additional set of driven wheels.
As a general rule, the hydraulic assist motors employed in such vehicles are subject to damage if the pistons of the motors are not retracted when the motors are de-activated and the vehicle is in motion, e.g., traveling on a paved road. While various hydraulic circuits are known for use with such selectively actuated hydraulic motors, such circuits often rely on highly complex valves that require extensive custom machining and, thus, can be quite expensive.